FOTK: Help with polyurethane casting

[ZombieGraves]

Hey all, currently I'm in the process of creating some polyurethane pieces of armor for a First Order Stormtrooper suit, and I've run into a bit of a snag. This is my first time casting with polyurethane, and I'm getting some really large bubbles in the surface, and a pretty uneven surface to begin with.
This was slush cast with Smooth On KX Flex 90 in a hard 3d printed resin mold. When I first demolded it after 30 minutes or so, it looked great, so I put it back in the mold to harden over the next week. I did a test piece before this, and that piece looked fine at first too, but over the rest of that week it started producing these large bubbles. Not like champagne bubbles, the surface is free of those, but air bubbles over an inch in diameter. When I took the full cast out of the mold after a week, that cast was suffering from the same thing. Popping the air bubbles on the inside of the cast with a razor blade helped them dissipate, but more kept appearing and moving around under the surface over time. KX Flex 90 is supposed to be fully cured after 7 days, but even now, a couple weeks after the casting, new bubbles are still appearing.
I haven't found any threads from anybody with the same experience. Does anybody have any suggestions? I'm at a loss here.

 

[drftfan]

A. Are you making sure to mix it properly and have the correct weighted parts?
B. What are you casting into?
C. Are you running and leaving it to cure in a warm space?
D. Are you using wooden sticks?

 

[ZombieGraves]

A. Are you making sure to mix it properly and have the correct weighted parts?
B. What are you casting into?
C. Are you running and leaving it to cure in a warm space?
D. Are you using wooden sticks?

A: Yep, mixed it correctly, and I'm using 100:120 as my ratio. Maybe I could be doing better if I got a digital scale though?
B: I'm casting into a cured resin print with smooth on spray mold release. Doing the spray brush spray method as it instructs.
C: I'm leaving it to cure in my living room, which never really drops below 65 at the lowest temp. I've heard that it might cure better if I put it into the oven, but I don't know if that's true or not.
D: Yeah, that's a piece of information I ran across after I did the cast. I've been using wooden sticks for my mixing, which I know now can introduce moisture to the mix, but I'd be shocked if that was the culprit, considering the bubbles are still appearing weeks after the initial cast.
What kind of cheap disposable stirring sticks would you recommend other than wood?

 

[Cbstudios]

My first instinct is that its moisture, but a couple of things to consider here.

Release agent - Are you waiting at least 30 minutes after the last application before you run your resin? its pretty unlikely to be release agent if its bubbling internally but could be that.

Resin - How old is your resin/how long has it been since initially opened? Older resins can have weird reactions, anything that's been opened for longer than maybe 9 months can start to have strange results depending on brand and type of resin. May be worth really thoroughly shaking both halves to make sure its not settling in the containers and separating.

Measuring - SUPER IMPORTANT, does your resin have specifics for measuring? being measured by weight versus by volume can have DRASTIC changes in results. If it specifies to be measured by weight, then weigh each half properly. Stick by the ratios. Your MSDS and MDS should have specifics for what ratios each method needs. Also really make sure that everything is mixed really thoroughly. Are you mixing any fillers like q-cell or microballoons into your resin? I'm guessing you're not pigmenting your resin but some pigments could react as well.

Finally, how long are you letting it set in the mold? you could let the pieces sit for the full cure period and see if you're still getting the bubbling on the surface, or if they migrate to the inside leaving the outside flat. Less ideal but could be a work around

 

[ZombieGraves]

My first instinct is that its moisture, but a couple of things to consider here.

Release agent - Are you waiting at least 30 minutes after the last application before you run your resin? its pretty unlikely to be release agent if its bubbling internally but could be that.

Resin - How old is your resin/how long has it been since initially opened? Older resins can have weird reactions, anything that's been opened for longer than maybe 9 months can start to have strange results depending on brand and type of resin. May be worth really thoroughly shaking both halves to make sure its not settling in the containers and separating.

Measuring - SUPER IMPORTANT, does your resin have specifics for measuring? being measured by weight versus by volume can have DRASTIC changes in results. If it specifies to be measured by weight, then weigh each half properly. Stick by the ratios. Your MSDS and MDS should have specifics for what ratios each method needs. Also really make sure that everything is mixed really thoroughly. Are you mixing any fillers like q-cell or microballoons into your resin? I'm guessing you're not pigmenting your resin but some pigments could react as well.

Finally, how long are you letting it set in the mold? you could let the pieces sit for the full cure period and see if you're still getting the bubbling on the surface, or if they migrate to the inside leaving the outside flat. Less ideal but could be a work around

I'm using the spray on version of this: Universal™ Mold Release Product Information
The instructions say to let it sit for 5 minutes after second application, so I've been waiting 5-10 minutes to be safe.

The polyurethane I've been casting with is brand new, I opened it the same day as the cast.

For measuring, I thought I was hitting that mark pretty well, but I was doing by eye. I just bought a scale so I can now measure it by weight rather than volume. Maybe that will help? The only thing I'm mixing in is some white pigment, also by smooth on. Perhaps I should more exactly measure that?

The cast was demolded after the recommended 30 minutes to make sure the mold release worked properly, then I put it back into the mold for the full cure time so that it would keep its shape as it hardened.

Judging by what I've read so far, my mixing/measuring might be the culprit here. Any good alternatives to wood stir sticks?

 

[drftfan]

Get a cake spatula. I use metal ones. They are easy to clean as well.
Yes, use a scale. Also measure out the tint.
The resin mold “may be” causing cure inhibitions but not likely.
And try to let it cure at room temp. I wouldn’t put it in the oven personally but doing a small wood box with a simple 100 watt lightbulb can help.

 

[ZombieGraves]

These are good ideas, thank you very much!